Short interfering RNAs (siRNAs) are small RNA regulators capable of reducing protein expression of their target mRNAs in a process known as RNA interference (RNAi). RNAi is important for normal cellular physiology and adaptation, as well as an invaluable genetic tool for scientists and a promising source of clinical therapeutics. While canonical models of RNAi feature siRNA-directed target mRNA cleavage by Argonaute proteins (Agos), recent studies have demonstrated that Ago-mediated cleavage is dispensable for target mRNA repression. Additional studies and comparisons with micro RNAs suggest that siRNAs may repress their target mRNAs via Ago-dependent translational repression and/or stimulation of degradation through non-Ago nucleases. To determine the mechanism(s) of siRNA-mediated mRNA repression, I propose a two-pronged approach: (1) a thorough analysis of siRNA-dependent changes in translation and degradation for siRNA target mRNAs in vivo and (2) biochemical purification and characterization of Ago-siRNA complexes and their interacting protein partners. I will utilize high throughput sequencing technologies to test severa different models of siRNA-dependent effects on translation and degradation in both C. elegans and H. sapiens, and take advantage of the wealth of functional genetic information in C. elegans to determine the cellular machinery required for any observed effect(s). To complement this approach I will directly identify the complexes associated with mRNA repression and assess their contributions to mRNA repression, again exploiting C. elegans' genetic resources. These approaches will provide a more detailed and comprehensive view of siRNA- dependent mechanism(s) of mRNA repression, and the techniques employed will be applicable to siRNA studies in many systems. Ultimately, knowledge of siRNAs' molecular effectors and mechanism(s) will be necessary for harnessing RNAi's full clinical and research potential, understanding the biological basis of off- target effects, and furthering our understanding of small RNA biology.